It’s got nothing to do with height. It’s a Chicago police code for murder. The rapper whose song this was taken from is from Chicago and the the context in which it appears in those lyrics make it clear it’s also about murder.

The 6’7” thing was made up by people trying to find reason or rhyme as to why a shibboleth they didn’t know would be said at a basketball game and inventing that it had to be connected to the height of one player.